Everyone studying the EIGRP details knows the “famous” composite metric formula, but the recommendation to keep the K values intact (or at least leaving K2 and K5 at zero) or the inability of EIGRP to adapt to changing load conditions is rarely understood.

IGRP, the EIGRP’s predecessor, had the same vector metric and very similar composite metric formula, but it was a true distance vector protocol (like RIP), advertising its routing information at regular intervals. The interface load and reliability was thus regularly propagated throughout the network and so it made sense to include them in the composite metric calculation (although this practice could lead to unstable or oscillating networks).

EIGRP routing updates are triggered only by a change in network topology (interface up/down event, IP addressing change or configured bandwidth/delay change) and not by change in interface load or reliability. The load/reliability numbers are thus a snapshot taken at the moment of the topology change and should be ignored.

You might be wondering why Cisco decided to include the load and reliability into the EIGRP vector metric. The total compatibility of IGRP and EIGRP vector metrics allowed them to implement smooth IGRP-to-EIGRP migration strategy with automatic propagation of vector metrics in redistribution points, including the IGRP-EIGRP-IGRP redistribution scenario used in IGRP-to-EIGRP core-to-edge migrations.

IP host (workstations, servers or communication equipment) is multihomed if it has more than one IP address. An IP host can be multihomed in numerous ways, using one or more layer-3 interfaces for network connectivity. Some multihoming scenarios are well understood and commonly used, while others (multiple physical layer-3 interfaces in the same IP subnet) could be hard to implement on common operating systems.

Cisco IOS allows up to 32 routing protocols contributing routes into a routing table (two of them are always connected and static). The limitation applies to the global routing table as well as to each individual VRF; the architectural reason for the limit is a 32-bit mask that’s used in Cisco IOS to mark individual routing protocols. The routing protocol ID (as displayed by the show ip protocol summary command) is thus limited to values 0 to 31. With value 0 being reserved for connected routes and value 1 for static routes, 30 values are left to number the routing protocols.

Due to the implementation details of Cisco IOS, the BGP, RIP and each EIGRP routing process consume routing protocol ID in all VRFs (regardless of whether they are used or not). You can view the IDs of individual routing protocols with the show ip protocol [vrf name] summary command.

Vijay sent me an interesting EIGRP query:

I know EIGRP hello packets are used to discover and maintain EIGRP neighborship and when an EIGRP router doesn’t receive a hello packet from its neighbor within the Hold timer, that router will be declared dead. But when would EIGRP declare a neighbor dead after sending 16 unicast packets?

The primary mechanism to detect EIGRP neighbor loss is the hello protocol. It’s a bit unreliable as it does not detect unidirectional communication, but has an interesting advantage that you can use asymmetrical hello/hold timers (each router can specify what hold timer its neighbors should use for its hello packets).

Every now and then, a really interesting question appears on the cisco-nsp mailing list. A while ago I’ve seen this one:

I’ve heard that Cisco devices handle ICMP at a low priority. I found one post describing it handled in process-switching and not fast-switching. Does anyone have an article that explains that process and is it configurable?

Most packets sent to the router are handled in process switching (the packet is queued in the input queue of one of the IOS processes), the obvious exceptions being GRE and IPSec packets (unless they’re fragmented).

Packets sent to the router can also be rate-limited with a control plane policy.

The IOS processes perform their job between interrupts (packets being CEF- or fast switched). A reply to an ICMP packet is therefore a lower-priority task than regular packet forwarding.

In 2007 and 2008, I wrote several articles covering small-site multihoming (a site connected to two ISPs without having its own public address space or running BGP).

Basics

A multihomed site is a customer site connected with (at least) two uplinks to one or more Internet Service Providers (ISP). Traditionally, a multihomed site needs its own provider independent (PI) public IP address space, has to run BGP with the upstream ISP and thus needs its own BGP autonomous system (AS) number.

Here’s a weird requirement that you could get on a really hard CCIE preparation lab (and hopefully never in a live network): redistribute external OSPF routes from selected ASBRs into BGP without using a route map on the redistribution router.

For example, assuming R1 and R2 insert external routes into OSPF, you want only routes from R1 to be redistributed into BGP on R3, but you cannot use route maps on R3.

I have to admit I have no hands-on Service Provider IPv6 experience (but then there are not too many people that can claim they do) and I don’t attend RIPE meetings, so I might have a completely wrong impression, but here it is: Is it just my perception or do we really lack any production-grade means of end-user multihoming in IPv6?