[笔记]Administrative Distances

SpookFox

一、Administrative Distances

The administrative distance (AD) is used to rate the trustworthiness of routing information received on a router from a neighbor router. An administrative distance is an integer from 0 to 255, where 0 is the most trusted and 255 means no traffic will be passed via this route.

If a router receives two updates listing the same remote network, the first thing the router checks is the AD. If one of the advertised routes has a lower AD than the other, then the route with the lowest AD will be placed in the routing table.
If both advertised routes to the same network have the same AD, then routing protocol metrics (such as hop count or bandwidth of the lines) will be used to find the best path to the remote network. The advertised route with the lowest metric will be placed in the routing table. But if both advertised routes have the same AD as well as the same metrics, then the routing protocol will load-balance to the remote network (which means that it sends packets down each link).

Table 5.2 shows the default administrative distances that a Cisco router uses to decide which route to take to a remote network.


If a network is directly connected, the router will always use the interface connected to the network. If an administrator configures a static route, the router will believe that route over any other learned routes. You can change the administrative distance of static routes, but, by default, they have an AD of 1.
If you have a static route, a RIP-advertised route, and an IGRP-advertised route listing the same network, then by default, the router will always use the static route unless you change the AD of the static route.
一些其它路由协议AD补充：
IS－IS      105
BGP         20 200



二、Routing Protocols


There are three classes of routing protocols:

Distance vector The distance-vector protocols find the best path to a remote network by judging distance. Each time a packet goes through a router, that’s called a hop. The route with the least number of hops to the network is determined to be the best route. The vector indicates the direction to the remote network. Both RIP and IGRP are distance-vector routing protocols.They send the entire routing table to directly connected neighbors.

Link state In link-state protocols, also called shortest-path-first protocols, the routers each create three separate tables. One of these tables keeps track of directly attached neighbors, one determines the topology of the entire internetwork, and one is used as the routing table. Linkstate routers know more about the internetwork than any distance-vector routing protocol.
OSPF is an IP routing protocol that is completely link state. Link state protocols send updates containing the state of their own links to all other routers on the network. Hybrid Hybrid protocols use aspects of both distance vector and link state-for example,EIGRP.


1、Distance-Vector Routing Protocols

The distance-vector routing algorithm passes complete routing table contents to neighboring routers, which then combine the received routing table entries with their own routing tables to complete the router’s routing table. This is called routing by rumor, because a router receiving an update from a neighbor router believes the information about remote networks without actually finding out for itself.

It’s possible to have a network that has multiple links to the same remote network, and if that’s the case, the administrative distance is checked first. If the AD is the same, the protocol will have to use other metrics to determine the best path to use to that remote network. RIP uses only hop count to determine the best path to a network. If RIP finds more than one link to the same remote network with the same hop count, it will automatically perform a round-robin load balancing. RIP can perform load balancing for up to six equal-cost links
(four by default).

However, a problem with this type of routing metric arises when the two links to a remote network are different bandwidths but the same hop count. Figure 5.9, for example, shows two links to remote network 172.16.10.0.Since network 172.16.30.0 is a T1 link with a bandwidth of 1.544Mbps, and network 172.16.20.0 is a 56K link, you’d want the router to choose the T1 over the 56K link, right? But because hop count is the only metric used with RIP routing, the two links would be seen as being of equal cost. This little snag is called pinhole congestion（针孔拥塞）.




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