Switches, Routers, Bridges and LANs/Routers/Border Router Selection

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On a perimeter router there are generally two border routers to avoid single point of failure. Interior routers and hosts on the perimeter network choose a border router to deliver their Internet traffic.

Central Question in Border Router Selection

Now we will see how reliable internet connectivity is establish with at least one working border router with reliable connection. Reliability, complexity, and hardware requirements can be traded off to meet the needs.

Border Router Selection vs. Exit Selection[edit | edit source]

Exit selection is the process used by BGP to decide which exit from your AS will be used. Border router selection is the process your interior routers and hosts use to pick a border router. Border router selection happens first as a host or interior router must choose a border router. Then the chosen border router decides if the packet should exit through one of its connections or if it should instead be forwarded to another border router for delivery.

Border Router Selection with IBGP[edit | edit source]

If all interior routers and hosts on the perimeter network run IBGP with the border routers then the border router selection problem can be neatly solved. Selecting Border Router with IBGP

The copy of BGP routing tables from each border router is coped into the interior router and host by IBGP. The interior router and host would always pick the best border router for each destination as they learned via IBGP. A lot of complexity is added to most hosts because of BGP. The extra memory and CPU power required by BGP in interior routers may make them substantially more expensive than they'd be if they didn't run BGP. Hence, most network designs will run BGP only on the border routers and therefore be faced with the border router selection problem. Now we will discuss network policies for selecting border routers without using BGP. Border Router Selection with a Static Route The simplest way for a host or interior router to choose a border router is to use a static default route. Static routing may lead to “wrong” border router selection.

Host Choosing "Wrong" Border Router[edit | edit source]

Consider network had default static route pointing at Border RouterB and host wanted to deliver traffic to a customer of ISPA. ISPA was sending customer routes so that your AS was aware that the destination was a customer of ISPA. In this case, Border RouterB would have learned ISPA's customer routes via IBGP from Border RouterA. So Border RouterB would receive the traffic and immediately redirect or forward it to Border RouterA via the perimeter network. The traffic would've traversed the perimeter network twice, wasting bandwidth. But if Border RouterB fails then there's an even higher price to pay. Host Unreachable from Internet when Border Router Fails

The interior router would likely share an IGP with the border routers and your IGP should be configured to select a functioning border router with at least one good Internet connection. Your IGP would detect the failure of Border RouterB so your interior router would use Border RouterA as its default route. It has a static default route pointing at the now dead Border RouterB. Hence it has lost all Internet connectivity. This is another example of how static routes and reliable networks often don't mix.

Border Router Selection with HSRP[edit | edit source]

With the help of two or more routers can dynamically share a single IP address. Hosts that have static default routes pointing at this address will see a reliable exit path from your AS without having to listen to BGP or your IGP. HSRP isn't a routing protocol at all. It's simply a way for routers on the same multi-access network to present a "non-stop" IP address. HSRP has the benefit that it keeps host configuration simple—a commonly used static default is all that's required. It also reacts to failures in a matter of seconds. Here are some examples of HSRP in action. HSRP with Two Border Routers in Normal Operation

The site has a T3 for its primary Internet connection and a T1 on a different border router for a backup. The perimeter network interface of Border RouterA is configured to have address The perimeter network interface of Border RouterB is configured to have address Since Border RouterA has the primary Internet connection, HSRP on it is configured so that it normally also holds the shared virtual interface address ( on its perimeter network interface. HSRP on Border RouterB is configured to monitor the health of Border RouterA. Internet traffic from the host follows the static default route toward to Border RouterA and exits on the T3 when both border routers are operating. But suppose Border RouterA fails

HSRP with Failed Primary Border Router[edit | edit source]

Within seconds of Border RouterA's failure, Border RouterB's perimeter network interface takes over the shared virtual interface address ( The static default route in the host now points to Border RouterB with no work on the host's part. Its Internet traffic now exits on the T1 via Border RouterB. Now suppose that the T3 fails but Border RouterA continues to operate. We want Border RouterB to take over the shared virtual address even though Border RouterA is still functioning. This case is handled by configuring Border RouterA to "give up" the address whenever it looses carrier detect on the T3.

HSRP with Failed Primary Internet Connection[edit | edit source]

This behavior is implemented with a priority system. Border RouterA is configured to lower its priority whenever carrier detect is lost on the T3. Border RouterB seizes control of the shared virtual interface address whenever it notices that its priority is now the highest in the group of routers sharing the address. (Yes, more than two routers can share a single virtual interface address.)

Limitations of HSRP[edit | edit source]

  • HSRP won't help you if an interface fails to pass data but carrier detect doesn't drop. This type of failure can happen if line between you and the central office is good but the DAX at the CO fails. BGP will eventually notice this kind of failure and reroute your traffic—it just won't happen with the speed of HSRP.
  • HSRP won't help your hosts pick the "optimal" border router. Note that HSRP is available on all Cisco routers, but can have only a single IP address on the lower-end routers (e.g. 1600, 2500, 2600, and 3600 series routers as of this writing).
  • HSRP can appear to interfere with outbound load sharing if you're not taking at least customer routes from one of your ISPs.
  • HSRP alone isn't sufficient for reliable Internet connectivity. You'll still need to have BGP configured correctly at your border routers and at all your ISPs to retain connectivity in the face of line and/or router failure.

Border Router Selection with Hosts Listening to IGP[edit | edit source]

HSRP is usually the best way for hosts to select a border router because it recovers quickly from failures and keeps host configuration simple. If you can't use HSRP, the next best choice for selecting a border router is to have hosts that listen to an IGP. It's most common for hosts to be able to listen to RIP, but the slow (several minute) convergence time of RIP makes it a poor IGP for those interested in reliability. OSPF makes a much better IGP, but is substantially more complicated than RIP.

Border Router Selection and Load Sharing[edit | edit source]

HSRP does a lot for reliability, but it can work against outbound load sharing in some cases. (Unfortunately, these cases often occur at sites with 2 T1s and more than 1 T1s worth of output bandwidth.) Load Sharing with BGP but Without HSRP Since both ISPs are sending only default routes, each border router will use its Internet connection for all exit traffic it receives. If each host generates about the same amount of outbound traffic, reasonably good outbound load sharing is achieved. (This might be especially desirable if both hosts together generated more traffic than would fit on either Internet connection individually.) Although the outbound load sharing might be good with this configuration, your outbound traffic might be reaching its destination through some pretty circuitous paths. As a quick reminder, think about what happens to traffic from HostB that destined for a customer of ISPA. It would have to be carried by at least ISPB (and perhaps several other ASes) before reaching ISPA. If either Internet connection fails, BGP will lose the default route it had heard through that connection. Exit traffic sent to either router will eventually exit on the remaining (working) Internet connection.

Load Sharing with BGP and HSRP[edit | edit source]

There are two changes that could be made to achieve both reliability and good outbound load sharing: •The border routers running HSRP could receive at least customer routes from one ISP. But this might require more memory be added to your border routers. •More than one HSRP virtual interface address could be used. Higher-end Cisco routers can be configured with two virtual interface addresses on the same physical interface. One of these addresses could be configured to favor Border RouterA in the normal case while the other were configured to favor Border RouterB in the normal case. Both would be configured to use the remaining working connection in the event of failure. HostA and HostB would then be configured with static default routes toward different HSRP virtual interface addresses. But lower-end Cisco routers support only one HSRP virtual interface address per physical interface. Alternatives here would be upgrading to higher-end routers or using lower-end routers with 2 interfaces to split the perimeter network.