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An Enterprise has reasonably good control over its outbound IP routes. The engineers can
configure BGP to set and react to Weight, Local_Pref, and AS_Path length, manipulating
each to choose a different outgoing link or different router through which to forward
packets to the Internet.
An Enterprise has much less control over inbound routes: routes for packets coming back
toward the Enterprise. First, these inbound routes exist on routers that the Enterprise does
not own. Even if an ISP or set of ISPs can be convinced by engineers at the Enterprise to
make their routes toward an Enterprise take a particular path, technical issues may prevent
the design from being implemented. In particular, if the Enterprise’s public IP address
range is summarized, the companies that use addresses in that range may have competing
goals, so no policy can be applied to influence the best route.
However, several tools exist that allow some control over the last ASN hop between an ISP
and their Enterprise customer. This book examines one such tool, called Multi-Exit Discriminator
(MED), originally worked for a dual-homed design–that is, with a single ISP
but with multiple links to that ISP. MED was later expanded to support dual-multihomed
designs (2+ ASNs, 2+ links), relying on the concept that ISPs would work together. This
section examines the dual-homed case, with a single ISP.
MED Concepts
The name Multi Exit Discriminator actually describes its function to a great degree. With
a dual-homed design, at least two links exist between the Enterprise and the ISP. The Enterprise
can announce to the ISP a value (MED) that tells the ISP which path into the Enterprise
is best. As a result, the ISP can discriminate between the multiple exit points from
that ISP to the Enterprise.
Because MED lets the Enterprise ASN tell just the neighboring ASN which link into the
Enterprise to use, engineers typically use MED when advertising an Enterprise’s public IP
address space. Those inbound routes into the Enterprise from the ISP typically consist of
either one, or a few, public IP address ranges.
For example, consider a new network design as shown in Figure 15-10. In this case, the
Enterprise uses the same 128.107.0.0/19 public address range used in Chapters 13 and 14.
The Enterprise connects only to ASN 1 with a total of four physical links and three BGP
neighbors.
Step 5. Origin: Whatever the Origin is (I or ?), it should tie.
Step 6. MED: None of the other steps determined the best route, so now MED
takes effect.
Table 15-7 summarizes the key points about MED.
Key
Topic
Note: For those of you memorizing using the NWLLA OMNI mnemonic, MED is the M
in OMNI.
MED Configuration
MED configuration usually occurs on the routers in the AS that wants to control inbound
routes from the neighboring AS. As such, in the example design shown in Figure 15-10,
Routers E1 and E2 would configure MED. Example 15-9 shows E1’s configuration.
Example 15-9 MED Configuration on Router E1
route-map set-med-to-I1-1 permit 10
match ip address prefix only-public
set metric 10
!
route-map set-med-to-I1-4 permit 10
match ip address prefix only-public
set metric 20
!
ip prefix-list only-public permit 128.107.0.0/19
!
Table 15-7 Key Features of MED
Feature Description
Is it a PA? Yes.
Purpose Allows an AS to tell a neighboring AS the best way to forward packets into
the first AS.
Scope Advertised by one AS into another, propagated inside the AS, but not sent to
any other autonomous systems.
Range 0 through 4,294,967,295 (232 – 1).
Which is
best?
Smaller is better.
Default 0
Configuration Via neighbor route-map out command, using the set metric command inside
the route map.
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router bgp 11
neighbor 1.1.1.1 route-map set-med-I1-1 out
neighbor 192.168.1.2 route-map set-med-I1-4 out
Both the configuration and the show ip bgp command output refers to MED as metric.
Note that the route map in Example 15-8 uses the set metric command, rather than set
med (which does not exist). And as shown in I1-1’s output for the show ip bgp command
in Example 15-10, the output lists MED under the heading metric. In particular, note that
even the show ip route command lists the MED value in brackets as the metric for the
BGP route.
Example 15-10 BGP Table and IP Routing Table on Router I1-1
I1-1# show ip bgp 128.107.0.0/19
BGP routing table entry for 128.107.0.0/19, version 13
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x820
Not advertised to any peer
11, (aggregated by 11 128.107.9.1), (received & used)
11.11.11.11 from 11.11.11.11 (128.107.9.1)
Origin IGP, metric 10, localpref 100, valid, external, atomic-aggregate, best
I1-1# sh ip bgp 128.107.0.0/19 longer-prefixes
BGP table version is 13, local router ID is 1.1.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*> 128.107.0.0/19 11.11.11.11 10 0 11 i
I1-1# show ip route 128.107.0.0 255.255.224.0 longer-prefixes
! Legend omitted for brevity
Gateway of last resort is not set
128.107.0.0/19 is subnetted, 1 subnets
B 128.107.0.0 [20/10] via 11.11.11.11, 00:02:18
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