EIGRP の Metric 調整時の Feasible Distance の更新について

Tsuyoshi Niitani · ‎2012-08-31

はじめに

EIGRP では、Interface の Bandwidth や Delay を変更して Metric を調整する際、変更後の Metric が FD (Feasible Distance) より大きくなる場合、FD の更新が行われません。このドキュメントでは、設定変更時の Debug 出力とともにその動作を解説します。

構成

                   S2/0 -- delay 1000 -- S2/0
[R1]E0/0 --- E0/0[R2]                      [R3] lo0:10.1.1.1/32
                   S3/0 -- delay 2000 -- S3/0

R3 では loopback0 を network コマンドにより配信
※以下、出力例はすべて 15.1(4)M3 を使用

変更前の状態

R2 では S2/0, S3/0 それぞれから 10.1.1.1/32 についてのUpdate を受信していますが Delay の設定から、S2/0 経由が Routing Table に Install されています。

R2#sh ip eigrp topology 10.1.1.1/32
EIGRP-IPv4 Topology Entry for AS(1)/ID(192.168.1.2) for 10.1.1.1/32
  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2041856
  Descriptor Blocks:
  172.20.1.3 (Serial2/0), from 172.20.1.3, Send flag is 0x0
      Composite metric is (2041856/128256), route is Internal
      Vector metric:
        Minimum bandwidth is 1544 Kbit
        Total delay is 15000 microseconds
        Reliability is 255/255
        Load is 1/255
        Minimum MTU is 1500
        Hop count is 1
        Originating router is 10.1.1.1
  172.20.2.3 (Serial3/0), from 172.20.2.3, Send flag is 0x0
      Composite metric is (2297856/128256), route is Internal
      Vector metric:
        Minimum bandwidth is 1544 Kbit
        Total delay is 25000 microseconds
        Reliability is 255/255
        Load is 1/255
        Minimum MTU is 1500
        Hop count is 1
        Originating router is 10.1.1.1
R2#
R2#show ip route 10.1.1.1
Routing entry for 10.1.1.1/32
  Known via "eigrp 1", distance 90, metric 2041856, type internal
  Redistributing via eigrp 1
  Last update from 172.20.1.3 on Serial2/0, 00:01:06 ago
  Routing Descriptor Blocks:
  * 172.20.1.3, from 172.20.1.3, 00:01:06 ago, via Serial2/0
      Route metric is 2041856, traffic share count is 1
      Total delay is 15000 microseconds, minimum bandwidth is 1544 Kbit
      Reliability 255/255, minimum MTU 1500 bytes
      Loading 1/255, Hops 1
R2#

この時の R1 での EIGRP Topology の状況は下記のようになります。

R1#sh ip eigrp topology 10.1.1.1/32
EIGRP-IPv4 Topology Entry for AS(1)/ID(192.168.1.1) for 10.1.1.1/32
  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2067456
  Descriptor Blocks:
  192.168.1.2 (Ethernet0/0), from 192.168.1.2, Send flag is 0x0
      Composite metric is (2067456/2041856), route is Internal
      Vector metric:
        Minimum bandwidth is 1544 Kbit
        Total delay is 16000 microseconds
        Reliability is 255/255
        Load is 1/255
        Minimum MTU is 1500
        Hop count is 2
        Originating router is 10.1.1.1
R1#

この R2 上で S2/0 を経由する状態から、作業などの都合により S3/0 経由へと変更する必要が出てきたとします。この時、Delay を操作し経路を迂回させる手法が一般的ですが、実施する内容により FD が更新されるかどうかの結果が変わってきます。次からその詳細について紹介します。

変更により Metric (FD) が大きくなる場合

このような作業を実施する場合、迂回したい経路の Interface の Delay を大きく変更したくなりがちですが、このように変更した場合には FD が変わらないままとなります。ただし、Routing Table 上の経路はMetric が小さいものが使用され、Update も送信されますので、経路選択に悪影響は与えません。

Interface S2/0 の Delay を 1000 から 3000 へ変更した場合

R2#
R2#debug eigrp fsm
EIGRP Finite State Machine debugging is on
R2#debug ip eigrp 10.1.1.1 255.255.255.255
Target AS enabled on AS 1 10.1.1.1/32

R2#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R2(config)#int serial 2/0
R2(config-if)#delay 3000
R2(config-if)#end
R2#
04:52:24.295: EIGRP-IPv4(1): dest(172.20.1.0/24) not active
R2#
04:52:25.427: %SYS-5-CONFIG_I: Configured from console by console
R2#
04:52:34.215: EIGRP-IPv4(1): rcvupdate: 10.1.1.1/32 via 172.20.1.3 metric 2553856/128256 on tid 0
04:52:34.215: EIGRP-IPv4(1): Find FS for dest 10.1.1.1/32. FD is 2041856, RD is 2041856 on tid 0
04:52:34.215: EIGRP-IPv4(1):    172.20.1.3 metric 2553856/128256
04:52:34.215: EIGRP-IPv4(1):    172.20.2.3 metric 2297856/128256 found Dmin is 2297856
04:52:34.215: DUAL: AS(1) RT installed 10.1.1.1/32 via 172.20.1.3
04:52:34.215: DUAL: AS(1) RT installed 10.1.1.1/32 via 172.20.2.3
04:52:34.215: DUAL: AS(1) Send update about 10.1.1.1/32.  Reason: metric chg on tid 0
04:52:34.215: DUAL: AS(1) Send update about 10.1.1.1/32.  Reason: new if on tid 0
04:52:34.295: EIGRP-IPv4(1): dest(10.1.1.1/32) not active
04:52:34.295: DUAL: AS(1) rcvquery: 10.1.1.1/32 via 192.168.1.1 metric 4294967295/4294967295, RD is 2297856 for tid 0
04:52:34.295: DUAL: AS(1) Send REPLY(r1/n1) about 10.1.1.1/32 to 192.168.1.1 for tid 0
04:52:34.359: DUAL: AS(1) Removing dest 10.1.1.1/32, nexthop 192.168.1.1
R2#

EIGRP Topology の Entry 上では FD は変更前の 2041856 ですが、Composite Metric は更新されています。Routing Table 上も S3/0 経由になっていることが確認できます。

R2#sh ip eigrp topology 10.1.1.1/32
EIGRP-IPv4 Topology Entry for AS(1)/ID(192.168.1.2) for 10.1.1.1/32
  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2041856
  Descriptor Blocks:
  172.20.2.3 (Serial3/0), from 172.20.2.3, Send flag is 0x0
      Composite metric is (2297856/128256), route is Internal
      Vector metric:
        Minimum bandwidth is 1544 Kbit
        Total delay is 25000 microseconds
        Reliability is 255/255
        Load is 1/255
        Minimum MTU is 1500
        Hop count is 1
        Originating router is 10.1.1.1
  172.20.1.3 (Serial2/0), from 172.20.1.3, Send flag is 0x0
     Composite metric is (2553856/128256), route is Internal
      Vector metric:
        Minimum bandwidth is 1544 Kbit
        Total delay is 35000 microseconds
        Reliability is 255/255
        Load is 1/255
        Minimum MTU is 1500
        Hop count is 1
        Originating router is 10.1.1.1
R2#
R2#sh ip route 10.1.1.1
Routing entry for 10.1.1.1/32
  Known via "eigrp 1", distance 90, metric 2297856, type internal
  Redistributing via eigrp 1
  Last update from 172.20.2.3 on Serial3/0, 00:05:42 ago
  Routing Descriptor Blocks:
  * 172.20.2.3, from 172.20.2.3, 00:05:42 ago, via Serial3/0
      Route metric is 2297856, traffic share count is 1
      Total delay is 25000 microseconds, minimum bandwidth is 1544 Kbit
      Reliability 255/255, minimum MTU 1500 bytes
      Loading 1/255, Hops 1
R2#

Update を受信するため、R1 上の Topology Entry も下記のように更新されます。
（2297856 は R2 での更新後の Metric と同じになります）

R1#sh ip eigrp topology 10.1.1.1/32
EIGRP-IPv4 Topology Entry for AS(1)/ID(192.168.1.1) for 10.1.1.1/32
  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2323456
  Descriptor Blocks:
  192.168.1.2 (Ethernet0/0), from 192.168.1.2, Send flag is 0x0
      Composite metric is (2323456/2297856), route is Internal
      Vector metric:
        Minimum bandwidth is 1544 Kbit
        Total delay is 26000 microseconds
        Reliability is 255/255
        Load is 1/255
        Minimum MTU is 1500
        Hop count is 2
        Originating router is 10.1.1.1
R1#

変更により Metric (FD) が小さくなる場合

逆に、S3/0 の delay を小さくした場合にどうなるかを確認したのがこちらの例です。設定変更後に経路が S3/0 経由となるのは同じですが、こちらの場合は FD が更新されます。

R2#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R2(config)#int s3/0
R2(config-if)#delay 500
R2(config-if)#end
R2#
05:02:01.543: %SYS-5-CONFIG_I: Configured from console by console
R2#
05:02:10.447: EIGRP-IPv4(1): rcvupdate: 10.1.1.1/32 via 172.20.2.3 metric 1913856/128256 on tid 0
05:02:10.447: EIGRP-IPv4(1): Find FS for dest 10.1.1.1/32. FD is 2041856, RD is 2041856 on tid 0
05:02:10.447: EIGRP-IPv4(1):    172.20.1.3 metric 2041856/128256
05:02:10.447: EIGRP-IPv4(1):    172.20.2.3 metric 1913856/128256 found Dmin is 1913856
05:02:10.447: DUAL: AS(1) RT installed 10.1.1.1/32 via 172.20.1.3
05:02:10.447: DUAL: AS(1) RT installed 10.1.1.1/32 via 172.20.2.3
05:02:10.447: DUAL: AS(1) Send update about 10.1.1.1/32.  Reason: metric chg on tid 0
R2#
05:02:10.447: DUAL: AS(1) Send update about 10.1.1.1/32.  Reason: new if on tid 0
R2#
------------------------------------------------------------------------------------------------------------
R2#sh ip eigrp topology 10.1.1.1/32
EIGRP-IPv4 Topology Entry for AS(1)/ID(192.168.1.2) for 10.1.1.1/32
  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 1913856
  Descriptor Blocks:
  172.20.2.3 (Serial3/0), from 172.20.2.3, Send flag is 0x0
      Composite metric is (1913856/128256), route is Internal
      Vector metric:
        Minimum bandwidth is 1544 Kbit
        Total delay is 10000 microseconds
        Reliability is 255/255
        Load is 1/255
        Minimum MTU is 1500
        Hop count is 1
        Originating router is 10.1.1.1
  172.20.1.3 (Serial2/0), from 172.20.1.3, Send flag is 0x0
      Composite metric is (2041856/128256), route is Internal
      Vector metric:
        Minimum bandwidth is 1544 Kbit
        Total delay is 15000 microseconds
        Reliability is 255/255
        Load is 1/255
        Minimum MTU is 1500
        Hop count is 1
        Originating router is 10.1.1.1
R2#
R2#sh ip route 10.1.1.1
Routing entry for 10.1.1.1/32
  Known via "eigrp 1", distance 90, metric 1913856, type internal
  Redistributing via eigrp 1
  Last update from 172.20.2.3 on Serial3/0, 00:11:48 ago
  Routing Descriptor Blocks:
  * 172.20.2.3, from 172.20.2.3, 00:11:48 ago, via Serial3/0
      Route metric is 1913856, traffic share count is 1
      Total delay is 10000 microseconds, minimum bandwidth is 1544 Kbit
      Reliability 255/255, minimum MTU 1500 bytes
      Loading 1/255, Hops 1
R2#

全機器設定

!
hostname R1
!
interface Ethernet0/0
ip address 192.168.1.1 255.255.255.0
!
router eigrp 1
network 192.168.1.0
!
------------------------------------------------------------------------------------------------------------
!
hostname R2
!
interface Ethernet0/0
ip address 192.168.1.2 255.255.255.0
!
interface Serial2/0
ip address 172.20.1.2 255.255.255.0
delay 1000
!
interface Serial3/0
ip address 172.20.2.2 255.255.255.0
delay 2000
!
router eigrp 1
network 172.20.0.0
network 192.168.1.0
!
------------------------------------------------------------------------------------------------------------
!
hostname R3
!
interface Loopback0
ip address 10.1.1.1 255.255.255.255
!
interface Serial2/0
ip address 172.20.1.3 255.255.255.0
!
interface Serial3/0
ip address 172.20.2.3 255.255.255.0
!
router eigrp 1
network 10.0.0.0
network 172.20.0.0
!